1. Field of the Invention
The present invention relates to the art of papermaking. In particular, the present invention describes a method and apparatus for improving the web formation of fourdrinier screen laid paper.
2. Description of the Prior Art
The papermaking process generally comprises a series of drying steps whereby a continuous flow of wood fiber suspended as a dilute aqueous slurry called stock is consolidated, first, to a wet fibrous mat and finally, to a dry finished paper web.
The fourdrinier papermachine on which this drying sequence is performed comprises first, a wet or forming section followed by a press section and a dryer section. Depending on the product objective, the dryer section may be followed by a calender finishing section.
The forming section of a papermachine comprises a flow receiving vessel for the stock. This vessel, called a headbox, is provided along the bottom thereof with a narrow slit opening called the slice. The slice constitutes the flow regulation device for control of stock flow from the headbox onto a perforate belt called the fourdrinier screen. This screen is driven around a closed belt course which includes a substantially horizontal portion called the table. Stock flow from the slice onto the screen forms a standing pond of stock on the screen along the table. Before the pond is allowed to flow over the lateral edges of the screen, sufficient water is drawn from the stock through the screen by various suction devices located beneath the table. By this action, the fiber, which originally constituted only about 0.5% of the slurry on the dry weight basis, is poured upon the screen as a low viscosity fluid and is removed therefrom at the end of the table as a consolidated fibrous mat.
From the wet end forming section, the fibrous mat is carried into the papermachine press section where additional water is removed mechanically by the squeezing action of a series of low pressure roll nips to form a compacted paper web.
Following the press section, the web is directed into the dryer section comprising a series of heated drums over which the web is threaded. The dryer section removes remaining water from the web evaporatively.
Final caliper and surface finishing of the dry web is achieved in the calender section by a series of high pressure nip rolls.
For purposes of product quality control and machine adjustment, most modern papermachines are equipped with instruments, usually located after the dryer section, to monitor the web characteristics of moisture, basis weight and conditioned weight. Sensors for these instruments continuously reciprocate across the web in the cross-direction (CD) while the web travels longitudinally along the machine-direction (MD) therebeneath. When data from these instruments is visually displayed by a chart recorder or cathode ray tube, a CD profile of the measured characteristics is revealed.
The basis weight characteristic relates to the total mass of web material, water plus fiber per unit of web area; usually per 1000 ft..sup.2 or per ream (3000 ft..sup.2). Since the moisture content of the web is measured independently of the basis weight, these two characteristic values may be combined to derive the characteristic of conditioned weight which is the measure of fiber quantity, exclusive of water, present in the web per unit of area.
It is the objective of every papermaker to achieve a uniform distribution of conditioned weight and moisture throughout the web in both, the CD and MD directions thereof. However, the mechanics of variations in these characteristics are different with respect to the CD and MD directions.
In the MD direction, magnitude variations in conditioned weight are predominantly due to random occurrences of fiber floccing: in other words, uncontrolled consolidation of fiber groups occurring in the headbox or before. Poor fiber distribution due to floccing is manifested in the finished sheet by a mottled or splotchy appearance.
Conditioned weight variations that are stable as to magnitude and CD location, on the other hand, are predominately the result of CD variations in the headbox slice opening. This paper web defect is seen in the finished product as light and dark streaks of web density.
CD moisture variations that are stable as to magnitude and CD location are normally an additional consequence of an improperly adjusted slice opening. Under such circumstances, a stable, high moisture region in the CD moisture profile is also attended by correspondingly high basis weight and conditioned weight magnitude profiles. However, on occasion, a stable high moisture region in the CD profile will not be attended by a conditioned weight concentration. This is a circumstance more apt to be caused by turbulence and flow characteristics internal of the headbox. Accordingly, the condition may not be readily affected by manipulation of the slice opening.
It would seem that a zone in the finished paper web that is uniform as to conditioned weight would be acceptable notwithstanding moisture variations. However, localized moisture variations affect the dimensional stability of the finished web and for this reason, all increments of the web must be dried to a threshold minimum moisture content. Consequently, if the web is burdened with a narrow, high moisture zone along the center thereof, the lateral remainder of the web must be over-dried in order to drive the high moisture zone below the threshold minimum. The overall result of these circumstances is an inefficient expenditure and waste of thermal energy in the papermachine dryer section on 90% of the web area, for example, to dry 10% of the web to tolerance.
From the perspective of moisture concentrations unattended by conditioned weight concentrations, it would be of great value to a papermachine operator to selectively adjust the moisture distribution in the web prior to the dryer section but independently of the fiber distribution.
U.S. Pat. No. 3,407,114 discloses a prior art technique for correcting a poor CD fiber distribution condition whereas U.S. Pat. No. 2,951,007 discloses a technique of selectively adding water to a predominantly finished web having a poor CD moisture distribution condition. In addition, U.S. Pat. No. 1,989,435 teaches a technique of altering the cross-directional strength of a paper web by impinging the fourdrinier pond with a pair of CD oriented curtain sprays.
U.S. Pat. No. 3,989,085 of William E. Crosby, having the assignee in common with that of the present application, describes a method and general apparatus for correcting the formation profile of a cross-directionally located anomaly. Due to the relevance of William E. Crosby's disclosure to the present invention, the disclosure thereof is hereby incorporated by reference.
Generally, Crosby's method included an array of fluid spray sources disposed above and across the fourdrinier table to direct a linked series of CD elongated fan spray patterns of fluid into the pond at select CD locations.
The apparatus of W. E. Crosby's disclosure anticipated several techniques of volume control for the described formation profile correction method, the most basic being the manually adjustable flow control valves.
For the purpose of automatic control, variable volume pneumatic valves were implied. Automatic electrical flow control was described relative to solenoid operated valves which are normally considered as binary operators, such valves having only two operative positions of entirely closed or completely open. This type of control, however, prohibits volume control in the sense of regulation.
Although electrically operated proportional valves are available to the art for electric powered automatic flow regulation, expense greatly inhibits the use of such devices. Regardless of whether the flow control is manual or automatic, regulation in the manner described is accomplished by flow throttling which has the consequence of pressure reduction. Since spray impact velocity is a significant factor in this profile control, pressure reductions due to flow rate throttling have adverse effects on the profile correction objective.
Moreover, fixed orifice nozzles secured to the distal ends of the several CD distributed conduits downstream of each flow control valve are designed to issue a precise pattern at a particular combination of flow rate and pressure drop. If this combination is significantly changed, so, too, is the nozzle spray pattern. Accordingly, over a full range of flow rate variation, the spray fan width may change 50% or more. Consequently, at the upper end of the flow rate range, adjacent spray fans may overlap excessively thereby flooding the overlapped swath. On the lower end of the flow rate range, adjacent spray fans will not meet thereby leaving strips in the pond unaffected by the sprays.